FIG. 5 is a cross-sectional view of a conventional semiconductor pressure sensor 1. The pressure sensor 1 includes a pressure-sensing chip 2 that responds to an applied pressure with an electrical signal or a change in an electrical signal. A cap 3 is joined to a stem 4 to enclose the pressure-sensing chip 2 in a package. The pressure-sensing chip 2 includes a diaphragm 5 that flexes in response to applied pressure. The pressure-sensing chip 2 is a unitary body of a semiconductor material, such as silicon, and the diaphragm 5 in the body is formed by etching and removing part of the body from one of its surfaces. The pressure-sensing chip 2 includes, at the unetched surface, four resistors formed in the chip by conventional means, such as doping or ion implantation. Those four resistors are connected in a conventional bridge circuit by conventional metallizations. When the diaphragm flexes in response to applied pressure, the resistances of the resistors change, resulting in a change in an electrical signal across the bridge circuit. That bridge circuit electrical signal is sensed to determine the pressure applied to the diaphragm 5.
In the structure of FIG. 5, the pressure-sensing chip 2 is mounted on a base 6 which, in turn, is adhered to the stem 4 with an adhesive 7. The base 6 includes a central opening in communication with a pressure-introducing tube 8 that is part of the stem 4. The opening in the base 6 is communication with the volume between the diaphragm 5 and the base 6. Connections to the bridge circuit are made by wires 9 that are connected to leads 10 that are mounted in and electrically insulated from the stem 4. Conventionally, the cap 3 is attached to the stem 4 with an airtight seal and a chamber 11 between the cap and stem is evacuated to form a zero pressure reference. In that arrangement, the pressure sensor measures the absolute pressure of the volume in communication with the tube 8. Alternatively, if the chamber 11 is in communication with the atmosphere, the pressure sensor measures gauge pressure.
The semiconductor pressure sensor of FIG. 5 produces a relatively weak electrical signal in response to ordinary pressures. Therefore, it is necessary to provide amplification to produce a usable electrical signal. In FIG. 6, an assembly including the pressure sensor 1 and signal amplification is shown in a perspective view. FIG. 7 is a bottom view of the same assembly that is shown in FIG. 6. In that assembly, the pressure sensor 1 is mounted on a supporting base 12 which, in turn, is mounted to a thick film substrate 13. The leads 10 from the sensor are connected to an integrated circuit 14 mounted on the reverse surface of the substrate 13 by conductors 22 disposed on the rear surface of the substrate 13. The integrated circuit 14 amplifies the electrical signal produced by the sensor. Resistors, including a resistor 15, are disposed on the front surface of the substrate 13 for controlling the electrical characteristics of the assembly. The integrated circuit 14 is connected through the substrate 13 to the resistor network on the front surface of the substrate. Clip leads 16 connected to the resistor network and, thereby, to the integrated circuit 14 of the sensor 1 are disposed at edges of the substrate 13 for mounting and connection of the assembly to other circuitry.
In operation, the pressure to be sensed is supplied to the sensing chip 2 through the tube 8. In response to the pressure, the diaphragm 5 flexes so that the resistors formed in the diaphragm 5 are subjected to various compressive and tensile forces, changing their resistances. The electrical signal across the bridge circuit is indicative of those resistances and, therefore, the applied pressure. That signal is amplified by the integrated circuit 14 to produce a final output signal.
The conventional pressure sensor 1 employs a relatively large number of parts and, therefore, must be relatively large. In addition, the only temperature compensation, i.e., correction of the final output signal for changes in the temperature of the sensor 1, is provided in the assembly of FIGS. 6 and 7 by the integrated circuit 14. That integrated circuit 14 is mounted on the thick film substrate 13 at a distance from the pressure sensor 1 and the pressure-sensing chip 2. Because of this separation, the temperature of the integrated circuit 14 may be significantly different from the temperature of the pressure-sensing chip 2, resulting in inaccurate temperature compensation.